Error handling

When a method on an object fails, the object is put into
an error state. Subsequent operations on the object do
nothing. The status of the object can be queried with
a function like status().

Constructors, rather than
returning NULL on out-of-memory failure,
return a special singleton object on which all
operations do nothing. Retrieving the status of the
singleton object returns CAIRO_STATUS_NO_MEMORY

Is this going to apply to
cairo_surface_t as well?

What about cairo_copy_path_data()? It's probably going to
have to return NULL.

Errors propagate from object to object. Setting a pattern
in an out-of-memory state as the source of a
cairo_t puts the type into an error state.

Much of the above is not yet implemented at the time of
this writing

A language binding could copy the C approach, and for a
language without exceptions, this is likely the right thing
to do. However, for a language with exceptions, exposing
a completely different style of error handling for cairo
would be strange. So, instead, status should be checked
after every call to cairo, and exceptions thrown as necessary.

One problem that can arise with this, in languages
where handling exceptions is mandatory (like Java), is that almost
every cairo function can result in a status being set,
usually because of an out-of-memory condition. This could make
cairo hard to use. To resolve this problem, let's classify then
cairo status codes:

If we look at these, the
CAIRO_STATUS_NO_MEMORY
should map to the native out-of-memory exception, which could
happen at any point in any case. Most of the others indicate
programmer error, and handling them in user code would be
silly. These should be mapped into whatever the language uses
for assertion failures, rather than errors that are normally
handled. (In Java, a subclass of Error rather than Exception,
perhaps.) And CAIRO_STATUS_READ_ERROR,
and CAIRO_STATUS_WRITE_ERROR can occur
only in very specific places. (In fact, as described
in the section called “Streams and File I/O”, these errors may be
mapped into the language's native I/O error types.)
So, there really aren't exceptions that the programmer must
handle at most points in the Cairo API.